It is well known the fact of producing the aforesaid gears by means of the following operations: cutting teeth into a cylindrical workpiece made of soft metal (“green”) (performed by means of one or more subsequent processes, in particular hobbing or shaping and shaving) and hardening.
With a reference to the aforesaid operations, the hardening process inevitably leads to changes in the gear profile, lead and pitch in particular: it alters the tooth macro-geometry. Therefore, given that the geometrical and the surface quality of the gear teeth depend on the sequence of processes they undergo, hardened gears intended for a high-performance use are inevitably subjected to subsequent finishing operations, in particular grinding operations, in order to obtain the best possible surface quality and accuracy. For a complete levelling of all imperfections of the gear geometry from the machining of soft metal “green machining” and due to distortion during hardening a minimum total stock removal normal to the flank of the gear of q≥0.03×m [mm] is needed, wherein m is the module of the gear.
The grinding processes of the aforesaid hardened gears, besides producing chips, generate high amounts of heat. Only a minimum of this heat can be taken off by the chips and by the tool, and therefore lubricating oils or emulsions are used, aiming: to cool the component in order to avoid that the too hot gear gets damaged during the grinding process. In particular, if the gear gets too hot (“burns”), the hardening is ruined and the gear surface is no longer suitable for the required use.
However, the use of lubricating oil implies several disadvantages:                complex and large systems to supply oil to and to return oil from the machine, filter chips from the oil and cool the oil to a certain temperature.        management costs for keeping the area surrounding the machines clean;        additional equipment to clean remaining oil from the ground gears;        maintenance costs for replacing the lubricating oil;        disposal costs of used lubricating oil, mixed with metal shavings;        protection and health care costs for the operators from contacting and inhaling lubricating oil.        
In order to avoid the use of lubricating oil, dry hobbing processes of hardened gears are known. However, the surface quality obtained by these processes is not sufficient for automotive transmission applications. Moreover, also dry grinding processes with extremely long production times are known (for example such processes are disclosed in “Innovative Zahnradfertigung”, ed. Expert Verlag, 1986, ISBN-13: 978-3-8169-1871-4) but these processes are not suitable for the production of automotive transmission gears with very short machining times (few seconds).